Examples of ridge waveguide optical devices incorporated with diffraction gratings include a distributed-feedback laser (DFB) formed from a compound semiconductor. In recent years, improvements in laser characteristics of the DFB laser have been proposed, where a structure in which a coupling coefficient to determine the amount of feedback of a diffraction grating is allowed to distribute in a resonator direction is adopted. For example, suppression of hole burning in the axis direction and an improvement in a longitudinal mode stability in a high optical output have been proposed, where a structure in which a coupling coefficient is allowed to distribute in such a way as to become small toward the resonator center is adopted.
In order to suppress an occurrence of hole burning, it has been proposed that the width of a buried diffraction grating is gradually decreased toward the resonator center or the width of the buried diffraction grating is gradually increased toward the resonator center. Furthermore, it has also been proposed that the height of a buried diffraction grating is gradually increased toward the resonator center or the height of the buried diffraction grating is gradually decreased toward the resonator center.
It has also been proposed that a large threshold gain difference or gain difference between the main mode and the side mode is obtained by adopting a structure in which the coupling coefficient is increased at the resonator center and the coupling coefficients at both ends are decreased as compared with the coupling coefficient at the center.
In the case where a drive electrode is divided into three parts in a resonator direction and a DFB laser to modulate the injection current of the center electrode is used as an FM modulation light source, it has been proposed that the width of the spectral line is decreased by increasing the length of the resonator.
In actual production of such a device, in the case where the structure in which a coupling coefficient is allowed to distribute in a resonator direction is adopted, it is desirable that a difference in coupling coefficient be increased between a region where the coupling coefficient is increased and a region where the coupling coefficient is decreased in order to improve the device characteristics. That is, it is desirable that the contrast in the coupling coefficient be enhanced.
However, in the case where a diffraction grating is formed by forming unevenness on the surface of an InP substrate and burying the unevenness in a semiconductor layer, in order to increase the difference in coupling coefficient between the region where the coupling coefficient is increased and the region where the coupling coefficient is decreased, it is desirable that the depth of the diffraction grating is specified to be very small in the region where the coupling coefficient is decreased.
It is difficult to precisely stably produce such a very shallow diffraction grating. Therefore, variations occur in the coupling coefficient, device characteristics (here, the threshold of lasing) are fluctuated, and the yield is not good.
For example, in the case where the depth of the buried diffraction grating is changed, the width of the diffraction grating in the region where the coupling coefficient is maximized is formed in such a way as to become half the period of the diffraction grating (duty ratio is 50%). In the region where the coupling coefficient is small, the width of the diffraction grating is made larger than that (duty ratio is larger than 50%) or smaller than that (duty ratio is smaller than 50%). However, in order to increase the difference in coupling coefficient between the region where the coupling coefficient is increased and the region where the coupling coefficient is decreased, it is desirable that the width of the diffraction grating is made very large or very small in the region where the coupling coefficient is decreased.
In the case where the width of the diffraction grating is made very large, an opening portion of a mask to form the diffraction grating becomes very narrow and, thereby, formation of the diffraction grating by etching is difficult. On the other hand, in the case where the width of the diffraction grating is made very small, the width of the etching mask is also made very small, although precise, stable production of a mask having, for example, a several-percent width is difficult. Even when a diffraction grating having a very small width is formed, the diffraction grating may disappear when being buried. Therefore, stable formation of a buried diffraction grating is difficult and the yield is not good.
Accordingly, the present inventors propose an optical device having a structure in which a coupling coefficient of a diffraction grating is allowed to distribute in a resonator and being capable of increasing a difference in coupling coefficient between a region where the coupling coefficient is increased and a region where the coupling coefficient is decreased at a high yield.
The following are reference documents:                [Document 1] Japanese Laid-open Patent Publication No. 8-255954,        [Document 2] Japanese Patent No. 2966485,        [Document 3] International Publication Pamphlet No. WO 2009/116152,        [Document 4] G. Morthier and others, “A New DFB-Laser Diode with Reduced Spatial Hole Burning”, IEEE Photonics Technology Letter, vol. 2, No. 6, pp. 388-390, June 1990,        [Document 5] M. Matsuda and others, “Reactively Ion Etched Nonuniform-Depth Grating for Advanced DFB Lasers”, 3rd International Conference on Indium Phosphide and Related Materials, TuF. 4, Apr. 8-11, 1991, and        [Document 6] S. Ogita and others, “FM Response of Narrow-Linewidth, Multielectrode λ/4 Shift DFB Laser”, IEEE Photonics Technology Letters, vol. 2, No. 3, pp. 165-166, March 1990.        